Erecting finder



March 10. 1931. 3. MITCHELL Re. 17,995

ERECTING FINDER I Original Filed May 22, 1925 i I nz/entar 6' 6079916.IfZL' tcizell.

Reissued Mar. I 10, 1931 UNITED STATES PATENT OFFICE A. mTOHELL, 01 LOS.ANGELES, CALIFORNIA, ASSIGNOR, BY MESR'E ASSIGN- MENTS, TO MITCHELLCAMERA CORPORATION, A CORPORATION OF DELAWARE ERECTING FINDEROriginalllo. 1,887,080, dated October 9, 1928, Serial No. 110,866, filedMay 22, 1926. Application for reissue filed October 7, 1930. Serial No.487,075.

I Referring first to Fig. 1 I'show at a suit- This invention relates tofinding instru ments for cameras and the like. The invene tion is usefulas a'finding instrument in connection with other optical instruments,but as it has been particularly designed for use in connection withcameras it will be explained in that connection without, however,

limiting the invention to such-particular use. Particularly in motionpicture cameras an 10' accurate and reliable finder is desired, as

' rect right and left relation, so as to avoid all great accuracy isrequired in getting exactly the correct field of view on the film. It isalso highly desirable that the image in the" finding instrument be shownupright and in corpossible confusion in the mind of the operator. Anobject of the present invention is to provide a simple form of findinginstrument that will show the field of view without inversion orreversal and will show it of a size suitable for easy inspection. It isa further particular object to provide an optical structure simple andeffective in itself for erecting the image,.which would other 2 wise beinverted and trans osed right for by. the ordinary objective lens of then er. 1

Other objects and correspondin accomplishments of the invention will ebetter understood from a consideration of the following detaileddescription, reference-for I ing another mo ified form.

It will be understood, and particularly so from what I say hereinafter,that the speclfic and particular forms herein shownand described areintended merely to be typical and illustrative of my -inventi on, andnots cifically limitative thereof; the invention ing only limited as is setforth in the appended claims.

able translucent surface on which the image isreceived. This translucentsurface may be any surface suitable for the purpose, many such beingknown in the art. Without intending any limitation, 1 shall here referto it as a. ground glass; Preferably I form this ground-glass surface onthe front face of a lightcollecting or slightly magnifying lens 11. Thislens and its front image receiving shape, its proportions correspondingto the proportions of the picture being received in the camera to whichthe finding instrument is attached. The ground glass thus forms a 1frame showing the outlines of the image actu ally received in thecamera.

The ground glass 10 may be'mounted in any suitable tube or other housing12, which also carries the erecting objective combination 13. Thiscombination may be carried in a. separate casing 14, and part 14a ofwhich may be tubular 1n shape so that it may slide in the housing 12 toadjust the distance between the objective combination and the groundglass. It will be understood, however, that to adjust this focaldistance it is only necessary to move the ground glass and .face may beconveniently. rectangular in the objective combination relatively, thateither one may be moved with reference to the housing. To keep'theobjective combination always in one relative position (up-'- right, asshown in the drawings) a pin 146 may play in a longitudinal slot 140 asshown in Fig. 1..

The objective J combination comprises an objective made up of one or twolenses 15. For instance, the objective may be formed of two lenses 15 ofsuitable configuration to obtain an image ofthe size desired on groundglass 10; and the two lenses maybe aflixed, as b cementing, to theopposite plane faces 7 of t e prism. structure of the objectivecombinatiomas is shown in the various views. It

will also be well understood that a single lens could be used as anobjective, but that use of double lenses is usually preferable inanobjective for various well known reasons, and also for the reasonthat, in this case, the light beam through the prism structure ispractically a parallel beam and the prisms therefore need only be ofsuch size as to have reflecting faces substantially the same size as thelenses.

The function of the prisms in the objective combination-is to erect andrectify the image finally thrown upon the ground glass. The

form shown in Figs. 1, 2 and 3 consists of a right angled prism 20 towhose hypotenuse face are cemented two smaller right angled prisms 21and 22 in the arrangement and relation best shown in Fig. 2. Supposingthe object to be at 0, a light ray from the object traverses the frontlens 15, strikes the face 21a of prism 21 and is reflected downwardly tostrike one of the 45 faces of prism 20 and thence reflected to the other45 face of that prism and thence upwardly to strike the 15" face 22a ofprism 22 and thence rearwardly through'the rear lens 15 to ground glass10. The reflection through the prism structure causes a completereversal of the image from what it would be if it were transmitted bythe lens alone; the result is that the image at I is in correct relative.position,

being correct as for right. and left erect. a

In Fig. 3 Ishow the same system but with and being the addition of asimple magnifying glass 25 infront of groundglass 10, the realaiinage onground glass 10 being viewed through the magnifying glass 25. In thiscase the lens at 11 performs the function of a light collector forthemagnifying glass 25, giving good illumination over the whole image.

In Fig. 451 show an objective combination I 13a of a somewhat differentspecific type. -Here the two lenses 15 are cemented to the flat frontand reanfaces of a prism of special form, this prism having two upperangular reflecting faces 30 and two lower angular ie fleeting faces 31.This form of erecting prism is known as the Brashear-Hastings erectingprism. Its function is substantially the same as the prismatic systemshown in Fig. 2; and acting in combination with the lens '15 it obtainsthe same result as before described. InFig. 4 the dash-line 35 shows howa ray would proceed through prism structure without lens 15, indicatinghow the image, without lens 15, would be-inverted and reversed; anddotted-line 35a shows how the same ray proceeds through thecompletecombination including the lenses, indicating that the image is erect andin correct position right for left.

There are other prismatic systems that may.

' be used in connection with an objective lens for the purpose ofrectifying-and erecting the image; but I prefer to use a system of suchphysical proportions that it may be placed between the two lenses of theobjective 'without separating those lenses too far to prevent theiracting as a single objective lens. It 18, of course, well known that theseveral individual lenses of a complete objective lens may be designedso as to be separated somewhat from each other; and I make use of thispossibility in my present design and separate the two lenses. Theseobservations, of course, apply only to the use of an objective havingtwo lens glasses. If a single lens glass is used, that single glass maybe cemented to the front face of the prismatic system.

Also, for the purpose of this erecting obpect'ive combination, I preferto use a prismatic system which has flat parallel front and rear facesso that the objective lens glasses may be cemented directly to thosefaces. By so doing I provide a simple, convenient and compact erectinglens combination with the prismatic system between the two lens glasses.In such a position, as will readily be understood, the prismatic systemneed not be verylarge, as the pencil of light passing or the wholeobjective combination may be composed or formed up, in any suitable way.have, for instance, shown in Fig. 4 how all ,t e prismatic reflectingfaces maybe formed on a single piece of glass, Without the necessity ofcementing together the three prism pieces shown in Fig. 2. Likewise thelens or lenses 15, although conveniently cemented to the prism faces,maybe applied thereto in any other convenient and suitable manner, It isonly necessary, in the optical system,

that the lens elements be located at the end faces of the prismstructure.

I claim:

1. In an erecting finder, an erecting obpective combination .comprisinganerecting prismatic system having flat parallel front and rear faceseach substantially at right angles to the optical, axis of the system,and an objective 'lens comprising two lens glasses cemented respectivelyto said front and rear faces of the prismatic system, the two lensglassesof the objective acting as a single objective lens which wouldwithout the interpoe sition of the prismatic erecting system throw an'inverted and reversed image; and an image receiving surface spaced fromthe objective combination and on which surface the objective throwsitsima e.

2. In an erecting-fin er, an erecting objective combination comprisingan erecting prismatic system, and, an objective of two lenses secureddirectly to the front and rear faces of the prismatic system; the lensesbeing of such focal lengths as to act as a single objective with a focallength that throws an image behind-the rear lens; and an image receivingsurface behind the rear lens.

3. In an erecting finder, an'erecting objective combination comprisingan erecting prismatic system, and an objective of two lenses secureddirectly to the front and rear 5 faces of the prismatic system; thelenses being of such focal lengths as to act as a single objective witha focal length that throws an image behind the rear lens.

4, In an erecting finder, an erecting objective combination comprisingan erecting prismatic system having lenses at the front and rear endsthereof, the lenses being'of such focal lengths as to act as a singleobjective with a focal length that throws an image behind the rear lens;and an image receiving surface behind the rear lens.

5. In an erecting finder, an erecting objective combination comprisingan erecting prismatic system having lenses at the front and rear endsthereof, the lenses being of such focal lengths as to act as a singleobjective with afocal length that throws an image behind the rear lens.

GEORGE A. MITCHELL.

